The structure and operation of the superheterodyne receiver is well-known in the prior art, as described in Communication Systems (A. Bruce Carson, McGraw-Hill Book Company, New York, second edition, 1975, pages 207-211), the disclosure of which is incorporated herein by reference.
As described by Carlson, a superheterodyne receiver comprises a preselector filter, a radio frequency amplifier, a frequency converter, an intermediate frequency filter, an intermediate frequency amplifier, and a detector. The preselector filter passes frequencies in the desired band which are amplified by the amplifier. The filtered and amplified signal passes to the frequency converter which converts the signal to an intermediate frequency. The frequency converter includes a mixer and a local oscillator. The received signal is applied to a first input of the mixer. A locally generated wave produced by the local oscillator is applied to a second input of the mixer. The mixer combines the inputs to generate an intermediate frequency signal. The mixer output, which is a fixed frequency signal, is filtered by the intermediate frequency filter to remove noise and undesired signals and then amplified by the intermediate frequency amplifier before passing to the detector. The detector separates the modulating signal from the carrier wave. This basic radio architecture to is called a superheterodyne receiver.
In superheterodyne receivers, it is desirable to use a low intermediate frequency to make amplification easier and to allow use of high performance filters that suppress adjacent channel interference. However, if a low intermediate frequency is chosen, a problem arises because of image response. If the carrier is below the oscillator frequency (f.sub.c =f.sub.o -f.sub.i-f), then a frequency separated from the desired carrier frequency by two times the intermediate frequency will produce the same result since the sign of the difference is not significant. This frequency, known as the image frequency (f.sub.s), will produce a spurious response at the receiver. This unwanted image response must be suppressed in the preselector filter. Suppression of the image response is easier the larger the value of the intermediate frequency. Thus, in single heterodyne receivers, a compromise value is chosen for the intermediate frequency.
More complex designs known as double-conversion superheterodyne receivers are taught in the prior art to resolve the conflict between adjacent channel suppression and RF image rejection that is inherent in the single conversion heterodyne receiver. In a double-conversion receiver, two mixers are used which translate the received signal to two different intermediate frequencies. The received signal is translated by a first mixer to the receiver's first intermediate frequency. The output of the first mixer is then filtered to suppress the image frequency and amplified before it is passed to the second mixer. Signals output by the first intermediate frequency amplifier are translated to a second intermediate frequency by means of a second mixer. At the second intermediate frequency, signals are filtered to further suppress adjacent channel interference, further amplified, and then passed to the detector.
With this arrangement, the first local oscillator must have the tuning range and degree of tuning resolution needed to accommodate the channel structure of the signals to be received. Moreover, the stability and spectral purity of the first local oscillator are important factors in determining the receiver's performance. In practice, however, the spectral purity and the stability of a tunable local oscillator degenerate as its tuning range increases and its resolution becomes finer.
In response to this unfavorable tradeoff, prior art receiver design has evolved according to two approaches. In the first approach, the tuning range of the first local oscillator--and therefore the tuning range of the receiver--are limited to a relatively narrow band of frequencies over which stability and spectral purity can be ensured consistent with a given fineness of tuning resolution. To extend the tuning range, prior art teaches the use of a plurality of separate mixer-oscillator pairs, one pair for each frequency band of interest. In the second approach, the amount and complexity of the circuitry is increased thereby increasing the cost of the receiver.